1. Field of the Invention
The various embodiments of the present invention generally relate to cobalt alloys, methods of processing cobalt alloys, and articles of manufacture made therefrom. More particularly, certain embodiments of the invention relate to methods of processing cobalt alloys to increase the tensile strength, yield strength, hardness, wear resistance, and fatigue strength of the alloys. Certain cobalt alloys processed according to various embodiments of the present invention are suitable for use in articles of manufacture, such as, for example, articulating medical implants.
2. Description of Related Art
Cobalt alloys are useful in a variety of applications requiring high tensile and fatigue strength, and/or corrosion resistance. For example, although not limiting herein, applications for which the properties of cobalt alloy are particularly well-suited include medical prosthetic or implant applications. More specifically, the fatigue properties of cobalt alloys are desirable for use in implants subjected to cyclical loading, such as hip or knee joint implants; whereas the corrosion resistant properties of cobalt alloys are desirable for biocompatibility. More particularly, cobalt alloys comprising chromium and molybdenum alloy additions, which are commonly referred to as “cobalt-chrome-moly” or “CoCrMo” alloys, have been widely used in both cast and wrought forms to form the articulating component of both knee and hip joint replacements.
However, one shortcoming of implants made from such conventional cobalt alloys is that the implants can degrade during service due to wear. As used herein the term “wear” means deterioration of at least a portion of a surface due to material removal caused by the relative motion between the at least a portion of the surface and at least a portion of another surface or substance. For example, it is known that certain implant surfaces, or “wear surfaces,” are subjected to substantial wear during service. As used herein, the term “wear surface” means at least a portion of a surface that is subjected to wear.
The deterioration of implant wear surfaces can ultimately result in the need to replace the implant. One particular problem associated with the deterioration of implant wear surfaces is the generation of wear debris. As used herein, the term “wear debris” refers to material that is removed from the wear surface during wear. For example, in “metal-on-polymer” articulating joints (i.e., joints wherein a metal surface articulates over a polymer surface), polyethylene wear debris is a principal cause of failures requiring replacement of the implant device. Further, concerns have been reported regarding the long term effects on the human body of small, high surface area alloy wear debris generated from wear of “metal-on-metal” articulating joints (i.e., joints wherein a metal surface articulates over another metal surface) and the elevated serum cobalt and chromium levels observed from wear of such joints.
In addition, the design of implant devices can be limited by the properties of the material used to make the device. For example, in ball and socket joint implants, the range of motion between the ball and the socket may be limited if the implants are made from materials having a relatively low tensile and/or fatigue strength due to the large size of the implant required. In contrast, the same implant made using materials having higher tensile and/or fatigue strengths would allow for a larger margin of safety. Alternatively, the use of higher strength materials could allow for the development of smaller implants with a greater range of motion. Additionally, the use of higher strength materials can permit a device design incorporating a smaller ball size, thereby reducing the volumetric wear rate of a polyethylene cup in a metal-on-polymer joint.
While increasing the hardness of implant wear surfaces can reduce the occurrence of wear-related implant failure by resisting the generation of wear debris during service, attempts to increase the hardness of implants have generally focused on nitriding or coating the surface of the implants. For example, U.S. Pat. No. 5,308,412 to Shetty et al. describes a method of hardening the surface of a cobalt-chromium based orthopedic implant device. The implant device is hardened by exposure to molecular nitrogen gas or ionized nitrogen at a temperature ranging from 500° F. to 2400° F. for a time sufficient to permit the diffusion of nitrogen into the surface of the implant. The nitrogen diffusion results in a hardened diffusion layer and hardened outer surface layer. See Shetty et al. at col. 3, lines 21-28. According to Shetty et al., the Knoop hardness of the implant can be increased up to 5000 KHN depending upon the temperature, time, and nitrogen gas pressure used. See Shetty et al., at col. 6, lines 24-26. U.S. Pat. No. 5,180,394 to Davidson discloses orthopedic implants coated with a wear-resistant coating of zirconium oxide, nitride, carbide, or carbonitride. For example, a zirconium containing alloy surface layer can be applied to a conventional implant material and thereafter treated to form a diffusion-bonded layer of zirconium oxide on the surface of the implant. See Davidson at col. 9, lines 53-57.
Cobalt alloys having improved friction and fatigue properties have also been disclosed. For example, U.S. Pat. No. 6,187,045 B1 to Fehring et al. discloses a cobalt alloy having improved fatigue and friction properties with ultra high molecular weight polyethylene (or “UHMWPE”), which is commonly used, for example, to form socket portions of metal-on-polymer implant devices. In particular, the cobalt alloy of Fehring et al. is essentially free of carbides, nitrides, and sigma second phases that reduce the friction and fatigue properties of the alloy.
However, utilization of coatings on implant devices has not been widespread due to concerns about reliability, and there remains a need for improved cobalt alloys for orthopedic implants that can increase the service life of the implant and reduce the number of revision surgeries necessary to replace failed implants. In particular, it would be desirable to develop cost-effective methods of processing cobalt alloys to increase the tensile strength, yield strength, hardness, wear resistance, and fatigue strength of the cobalt alloys that can be used in conjunction with a variety of cobalt alloy compositions.